First, is the existing cable any good?
Aside from the obvious material condition of the wire...
Here's how "grandfathering" works. It is legal to continue an installation in service if it was legal when initially installed.
- all-insulated (white black red) no-ground NM cable was legal
- Service Entrance type (2 insulated blacks + a mesh neutral) was legal.
You were meant to switch to 3-wire+ground as soon as your supplier ran out of 3-wire/no-ground. Unfortunately what some people did was switch to 2-wire+ground (black white bare), and misuse the bare wire for neutral. That was never allowed. So if that's your cable, you can't use it. Leave it in the walls, since it CAN be used for a "separates" rangetop that does not require neutral. If someone installs "separates" range/oven, they'll thank you for leaving it there.
Ovens need neutral for the oven light. Back when incandescents were the only light, everybody had a drawer full of 120V ones, and oven builders wanted you to be able to just use those. That's moot nowadays since LEDs burn up in ovens.
The upshot is, if the cable isn't any good, it's a moot question - you'd need to replace it anyway.
Yes, retrofitting ground is allowed...
The wire currently in your 3-wire feed is neutral, not ground. Even if it is the bare mesh of SE cable, it is still neutral. (service entrances don't have ground).
A #10 ground wire can be retrofit via any reasonable route, and it doesn't even need to go back to the panel - it can reach any of
- a junction box with non-bendable metal conduit back to the panel
- a junction box with #10 or larger ground wire back to the panel
- straight to the Grounding Electrodes, the ground wires between panel and water pipe/ground rods. (note this cannot be cut; you use a split bolt).
... however, maybe not retrofitting and extension
To begin with, the extension would require a splice. The splice must occur inside a large-enough junction box, and the cover of the box must remain accessible forever without needing tools to disassemble any part of the building. It can be in the back of a kitchen cabinet if it's reachable for maintenance; it cannot be blocked in by a kitchen cabinet. So any solution that renders the splice outside a box, in a too-small box, or in an inaccessible box is a deal killer.
Second, the AHJ (local inspector) has to be OK with a retrofit as part of an extension. This is, at best, a gray area, and the inspector could rule either way, and knock you on your socks! I would run it by them before committing to it.
You can use a cable, but conduit is better
The absolutely ideal situation for this is a conduit running back to the panel. That would allow you to fit any appropriate wires at any time, so it makes it easy to, say, put in a 50A all-in-one and fit dual 30A separates later. It would also economize, because with the individual THHN wires used inside conduit, #8 copper THHN is good for 50A.
If you're willing to fit conduit, you'll thank yourself later. Non-flexible metal conduit will take care of grounding as well, but is somewhat more difficult to fit. 3/4" conduit should satisfy all requirements. The conduit must be installed, adequately stapled down and complete before any wires are pulled in; this often frightens novices but it really is the easier way to do it.
But feel free to use cable if that is easier.
NM and UF cables have current limitations
They are the "shoemaker's sons" of electrical cable, having a lower ampacity than their "big brother" cables (of every other type).
- #8 NM/UF is only good for 40A. #8 anything else is good for 50A.
- #6 NM/UF is only good for 55A. #6 anything else is good for 65A.
- #6 aluminum has the same ratings as #8 copper, except NM and UF are generally not offered in aluminum, so it's good for 50A.
"Other types" of /3+ground cable are hard to find... but in conduit country, you aren't meant to put cable in conduit, you use individual wires such as THHN or XHHW. They are allowed the higher ampacity.
Aluminum is fine for large feeder, just not for this
Apprehensions about aluminum generally are misplaced. If you knew the full story of aluminum wire, you'd cast the blame in quite different places; and in fact you'd run out and buy a torque screwdriver! But that aside...
Aluminum is not a good fit when the terminations are not rated for aluminum. So look at your range receptacle. If it is UL-listed for aluminum wire, then you're all set.
At the breaker end, it's not a problem; during the aluminum backlash they examined everything, and the standard for breaker connections passed, and that UL standard did not need to be revised.
